The invention relates to an exchanger for the transfer of heat between two circuits of a nuclear power station.
The invention more particularly relates to an exchanger, such as a steam generator or boiler having a casing in which the heat carried by a primary fluid is transferred to a secondary fluid in such a way as to heat and possibly vaporize the latter, the secondary fluid circulating in tubes, whose opposite ends issue into supply and discharge mains arranged externally of the steam generator casing. Steam generators of this type are used in particular, although not exclusively in the circuits of liquid metal-cooled nuclear reactors for ensuring a heat transfer between a cooling ciruit in which travels a liquid metal such as sodium and a circuit which supplies an electricity generation installation in which flows a liquid such as water.
In steam generator of this type, the length of the tubes in which the secondary fluid flows, said fluid generally being water, is adapted to the quality of the steam to be produced and is often approximately 100 meters. There is a relatively large number of these tubes in order to permit the maximum heat exchange between the liquid sodium which generally forms the primary fluid and the water or steam circulating in the tubes, the latter immersed in liquid sodium. For obvious safety reasons, it is vital to be able to check the sealing of the tubes. It is also necessary to be able to carry out certain measurements and in particular the measurement of the steam temperature on leaving the tubes, as well as certain checks during periods when the generator is shut down, such as the individual inspection of each of the tubes, for example by means of an ultrasonic or eddy current probe.
Steam generators are known in which the tubes issue at each of their ends into a water box and a steam box, which are substantially cylindrical, whereby for example there are four supply boxes and four discharge boxes whilst the generator has approximately 360 tubes, in such a way that approximately 90 tubes issue into each of the boxes. Mains collect together the outlets of the water and steam boxes. In order to permit the checks and measurements referred to hereinbefore, the tubes issue into one of the ends of each of the boxes via a removable cover, whose removal gives access to the orifice of each of the tubes issuing into the said box. Due to the number of tubes issuing into each of the boxes, the dimensions of the latter are relatively large in such a way that they must incrorporate tube plates and thick walls, whilst the cover diameter is relatively large. As a result, there can be sealing defects during operation between each box and its cover, which is conventionally in the form of an autoclave joint, whose diameter can reach approximately 420 mm. Moreover, as a result of the box dimensions, whose length is considerable compared with its diameter it is relatively difficult to mark the tubes and obtain access to a clearly defined orifice. Finally, the exchanger and mains are generally positioned within thermal insulation, which must be removed before access to the tubes can be obtained.